Method for determining pump flow rates using motor torque measurements

ABSTRACT

A method and system for determining fluid flow rates through a motor driven pump controlled by a variable speed drive is disclosed. The variable speed motor drive is used to characterize a pump fluid flow for a plurality of motor torque values at a plurality of known pump speeds. These characterized pump flow rate/torque values/speed are stored and used to determine a pump fluid flow at a measured pump speed and motor torque by interpolating between the known characterized fluid flow/torque/speed values. In another aspect of the invention, the pump flow rate at a measured torque, at a specified characterizing speed value is determined by interpolating between known characterizing pump flow rates. The determined pump flow rates can then be used to determine pump flow rates at a measured speed by interpolating between corresponding speed values which bound the measured speed.

FIELD OF THE INVENTION

[0001] This invention relates to fluid flow rate measurements. Morespecifically, this invention relates to determining fluid flow ratesusing measured motor speed and torque parameters.

BACKGROUND OF THE INVENTION

[0002] Measurement of fluid flow through pipes and pumps is well knownin the art. One method of determining fluid flow rates is to installgears, vanes, paddle wheel, turbines, etc., in the flow channel anddetermine fluid flow rate by the speed at which these devices turn. Asecond method is by measuring the differential pressure across adedicated flow obstruction, such as a venturi, orifice plate, annubar,pitot tube, etc., and applying the well-known Bernoulli's principle toobtain a velocity and, consequentially, a fluid flow rate. As an exampleof this principle, U.S. Pat. No. 5,129,264, entitled “Centrifugal Pumpwith Flow Measurement,” issued Jul. 14, 1992, to Lorenc, and assigned tothe same assignee herein discloses using differential pressure and pumpspeed to measure fluid flow rates. Still other methods of fluid flowrate measurement employ electrical/magnetic or sonic measurement means.For example, Mag Meters determine fluid flow rates by measuring thechange in a magnetic field caused by the velocity of the fluid flowingtherethrough. Sonic devices use acoustical pulses, i.e., Sonar, andDoppler principles to measure fluid flow rates. Other non-intrusivemethods measure the torque a variable speed electrical motor delivers toa pump by installing a torque shaft between the motor and pump. Themotor or pump is then calibrated and a motor kilowatt input/Motor BrakeHorsepower Output (BHP) calibration table is developed. Accordingly,knowledge of the kilowatt input value can be used to determine theoutput horsepower. However, this calibration is needed at several speedsand requires several different sized torque shafts.

[0003] Thus, current methods for determining fluid flow ratesnecessitate intrusion into, or require access to, the enclosurestransporting the fluid. In some systems, such as caustic systems havinglined pumps, intrusion is prohibited.

[0004] Hence, there is a need for a simple, accurate and reliable methodfor determining fluid flow rates without intruding into the fluid flowor having access to the enclosures transporting the fluid.

SUMMARY OF THE INVENTION

[0005] The present invention determines a fluid flow rate through a pumpby first determining two flow rate values from a plurality ofcharacterizing flow rate values corresponding to two known speed valuesselected from a plurality of known characterizing speed values at aknown motor torque. The first one of the two known speed values isselected greater than a measured pump speed and a second one of the twoknown speed values is selected less than the measured pump speed. Thefluid flow rate is then determined as being proportional to the twodetermined flow values at the known speed values and the pump speed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] In the drawings:

[0007]FIG. 1 illustrates a conventional pump driven by a variablespeed/motor drive system;

[0008]FIG. 2 illustrates an exemplary processing flow chart inaccordance with the principles of the present invention;

[0009]FIG. 3 illustrates a determination of a fluid flow rate using anexemplary torque vs. flow rate graph in accordance with the principlesof the invention; and

[0010]FIG. 4 illustrates a second determination of a fluid flow rateusing an exemplary torque vs. flow graph in accordance with theprinciples of the invention.

[0011] It is to be understood that these drawings are solely forpurposes of illustrating the concepts of the invention and are notintended as a level of the limits of the invention. It will beappreciated that the same reference numerals, possibly supplemented withreference characters where appropriate, have been used throughout toidentify corresponding parts.

DETAILED DESCRIPTION OF THE INVENTION

[0012] This invention applies to the ability of new generation variablespeed drives (VFD), such as the available ABB ACS601, to characterizethe motor that they drive and accurately determined the torque beingproduced by the motor to the pump. A motor may be characterized bydetermining a fluid flow rate through a pump at a plurality of knownspeeds and torque values. The characterization flow rate/torqueinformation is retained and used to determine fluid flow rate atmeasured, non-characterized, speed and torque values.

[0013]FIG. 1 illustrates an exemplary system configuration of a pumpingsystem 100. In this exemplary system, motor 120 communicates with pump110 through shaft 115 and produces a rotary motion in shaft 115 thatimposes a rotary motion on an impeller unit (not shown) within pump 110.The rotation of the impeller causes fluid within pump 110 to be expelledthrough an output orifice 130. Variable speed motor drive (VFD) 150provides control signals to motor 120, which control the speed of motor120.

[0014]FIG. 2 illustrates an exemplary flow chart of fluid flow ratedetermination processing in accordance with the present invention. Asillustrated, a motor torque measurement is obtained at block 210 and amotor speed is obtained at block 220. Motor torque and speedmeasurements using VFD 150 is well known in the art. For example, torquetransducers utilize resistive, photoelectric or strain gauge sensors,such as metal foil or piezo-resistive sources. The metal foil devicesare placed on a shaft of the motor and the leads from the device arecarried through the shaft to slip rings, which are brushes that areattached to monitor output terminals. The output measured at theterminals can be used to measure motor torque.

[0015] At block 230 a flow rate value is determined for a known speedvalue above the measured motor speed value corresponding to the measuredmotor torque value. At block 240 a flow rate value is determined for aknown speed value lower than the measured motor speed valuecorresponding to the measured motor torque value. At block 250 a fluidflow rate value is determined by evaluating a proportional relationbetween the obtained two fluid flow rate values, the corresponding speedvalues and the measured pump speed.

[0016] In one embodiment of the invention, an actual flow rate value isdetermined by performing a linear proportional evaluation, e.g., aninterpolation, in accordance with Equation 1: $\begin{matrix}{F_{x} = {F_{2} + \frac{\left( {S_{k} - S_{2}} \right)\left( {F_{1} - F_{2}} \right)}{\left( {S_{1} - S_{2}} \right)}}} & \lbrack 1\rbrack\end{matrix}$

[0017] Where F_(x) is the actual flow at a measured torque value;

[0018] S_(k) is the pump speed;

[0019] S₁ is a known speed greater than S_(k)

[0020] S₂ is a known speed lower than S_(k);

[0021] F₁ is the flow rate at known speed S₁ at a measured torque value;and

[0022] F₂ is the flow rate at known speed S₂ at a measure torque value.

[0023]FIG. 3 illustrates an example of the processing to determine fluidflow rate in accordance with the principles of the present invention. Inthis illustrated example, the graph 300 depicts Fluid Flow versus MotorTorque to show the characterization of fluid flow rates through a pumpat known motor torque values. In the example, this is shown threedifferent pump speeds. As would be appreciated, of different pump speedsused to characterize the fluid flow may be substantially increasedwithout altering the principles of the invention. In graph 300,measurements of torque and corresponding fluid flow rate are plotted foreach of the illustrated speeds, 310, 320, 330, respectively. Flowrate/torque values 312, 314, 316 etc., are representative of flowrate/torque values that characterize a motor performance at a firstspeed 310. Similarly, flow rate/torque values 322, 324, 326, etc., and332, 334, 336, etc., are representative of flow rate/torque values thatcharacterize a motor performance at the illustrated second and thirdspeed, respectively. Accordingly, there is a fluid flow rate valueassociated with each torque value at a specified speed.

[0024] Further illustrated, are dashed lines 318, 328 and 338, which arerepresentative of polynomial expressions that can be used to evaluateand determine pump fluid flow rates at measured, non-characterized,torque values at measured, non-characterized, pump speed. Althoughillustrated as high-order polynomial expressions, it is appreciated,that the polynomial expression may be selected as a linear, a piece-wiselinear, a quadratic expression, etc. A polynomial expression can beselected, for example, to minimize a mean square error among thecharacterizing flow rate/torque values at each of the specifiedcharacterizing speeds.

[0025]FIG. 3, further, graphically illustrates the determination of pumpfluid flow rate at a measured torque and speed in accordance with theprinciples of the invention. In this illustrated example, a measuredpump speed/motor torque value 350 is determined using known techniquesassociated with variable speed motor drives. Known characterized fluidflow/torque values, corresponding to the measured torque value, are nextobtained for a characterization speed value higher than the measuredspeed value and a characterization speed value lower that the measuredspeed value. In this case, characterization fluid flow/torque values 325and 335 are determined for a higher and a lower speed value,respectively.

[0026] An actual fluid flow rate value 350, associated with the measuredspeed/torque values can then be obtained by interpolating between thehigher speed fluid flow value 325 and the lower speed fluid flow value335. In one aspect of the invention, the interpolation algorithmutilized corresponds to a linear interpolation between the fluid flowrate/torque values determined at the higher and lower speed values.

[0027] Although FIG. 3 illustrates the principles of the invention, withregard to a measured torque value corresponding to a characterizingtorque value, it is understood that a polynomial expression may be usedto determine fluid flow rates at a designated characterizing speed whenthe measured torque does not correspond to one of the plurality ofcharacterizing torque values.

[0028]FIG. 4 illustrates a second aspect of the invention, whereincharacterization of fluid flow rates at non-measured fluid flow/torquevalues may be obtained by interpolating, for example, between knowncharacterization fluid flow/torque values. More specifically, aplurality of characterizing fluid flow/torque values are retained, forexample in a memory, for each of a plurality of characterizing speedvalues. For measured pump speed/torque value 350, a fluid flow rate isdetermined at a first characterization speed value by interpolatingbetween two characterization flow rate/torque values associated with thecharacterization speed. In this illustrated example, first fluid flowrate value 413 associated with a known characterization speed higherthan the measured speed/torque value 350 is determined by interpolatingbetween characterization values 412 and 414. Characterization values412, 414 are associated with torque values which bound the measuredtorque value. Similarly, a second fluid flow rate is determined at asecond characterization speed by interpolating between twocharacterization flow rate/torque values. In this illustrated example, asecond fluid value 423 associated with a known characterization speedlower than the measured speed/torque value 350 is determined byinterpolating between characterization values 422 and 424. These valuesare also associated with torque values, which bound the measured torquevalue.

[0029] A fluid flow rate corresponding to the measured speed/torquevalue 350 is then determined, for example, by interpolating between thedetermined two fluid flow values 413, 423, the two known characterizingspeed values, and the measured speed, as previously disclosed.

[0030] Although the invention has been described and pictured in apreferred form, it is, however, understood that the present disclosureof the preferred form, has been made only by way of example, and thatnumerous changes in the details may be made without departing from thespirit and scope of the invention as hereinafter claimed. It is intendedthat the patent shall cover by suitable expression in the appendedclaims, those features of patentable novelty that exists in theinvention disclosed.

I claim:
 1. A method for determining an actual fluid flow rate through amotor driven pump controlled by a variable speed drive at a measuredmotor torque value, comprising the steps of: receiving a measure of saidmotor torque and a measure of a motor speed; determining a flow ratevalue from a plurality of characterizing flow values at each of twoknown speed values selected from a plurality of known characterizingspeed values, wherein a first one of said two known speed values isselected from a known speed value greater than said measured speed valueand a second one of said two known speed values is selected from a knownspeed value less than said measured speed value; determining said actualfluid flow rate proportional to said determined flow rate valuecorresponding to each of said two selected know speed values, said twoselected known speed values and said measured speed.
 2. The method asrecited in claim 1 wherein said motor torque is determined by saidvariable speed drive.
 3. The method as recited in claim 1 wherein saidspeed is determined by said variable speed drive.
 4. The method asrecited in claim 1 wherein the step of determining said flow rate valuecorresponding to a selected one of said two known speed values,comprises: evaluating a known relation between at least two knowncharacterization flow rate values at said measured torque value.
 5. Themethod as recited in claim 4 wherein said known relation is a polynomialrelation.
 6. The method as recited in claim 5 wherein said polynomial isa linear polynomial.
 7. The method as recited in claim 4 wherein saidpolynomial is determined as having a minimum mean square error amongsaid characterizing flow rate values.
 8. The method as recited in claim4 wherein said known relation is piece-wise linear between each of twocharacterizing flow rate values among said plurality of characterizingflow rate values.
 9. The method as recited in claim 5 wherein saidpolynomial is a quadratic polynomial.
 10. The method as recited in claim1 wherein the step of a determining a flow rate value at a selected oneof said two known speed values comprises: extracting two flow ratevalues from a plurality of characterizing flow rate values, wherein afirst one of said two extracted flow rate values corresponds to a knownfirst torque value lower than said motor torque and a second one of saidtwo extracted flow rate values corresponds to a known torque valuegreater than said motor torque; and interpolating between said first andsaid second extracted flow rate values.
 11. The method as recited inclaim 1 wherein the step of determining said flow rate value at aselected one of said two known speed values comprises: extracting afluid flow rate value at said measured motor torque.
 12. The method asrecited in claim 1 wherein said proportional relation is a linearrelation.
 13. A system for determining an actual fluid flow rate througha motor driven pump controlled by a variable speed drive at a determinedmotor torque, comprising: a measuring means coupled to said motor todetermine said motor torque; a measuring means coupled to said motor todetermine a speed value; a memory retaining a plurality of knowncharacterizing flow rate values corresponding to a plurality of knowncharacterizing torque values for each of a plurality of knowncharacterizing speed values; a processor, in communication with saidmemory, operative to: receive said determined torque value and saiddetermined speed value; determine a flow rate value from said pluralityof characterizing flow values at each of two known speed values selectedfrom said plurality of known characterizing speed values, wherein afirst one of said two known speed values is selected greater than saidmeasured speed and a second one of said two known speed values isselected less than said measured speed; and determine said actual fluidflow rate proportional to said determined flow rate value at each ofsaid selected known speeds, said selected known speed values and saidmeasured speed.
 14. The system as recited in claim 13 wherein said motortorque means comprises: a torque measurement device selected from thegroup of resistive, photoelectric, strain gauge sensors.
 15. The systemas recited in claim 13 wherein said speed means comprises: an accurate(±10 rpm) means of pump speed measurement.
 16. The system as recited inclaim 13 wherein said processor is operative to determine said flow ratevalue at a selected one of said selected two known speeds by: evaluatinga known relation between at least two known characterizing flow ratevalues at said measured torque value.
 17. The system as recited in claim16 wherein said known relation is a polynomial relation.
 18. The systemas recited in claim 17 wherein said polynomial is a linear polynomial.19. The system as recited in claim 17 wherein said polynomial relationis determined as having a minimum mean square error among said pluralityof characterizing flow rate values.
 20. The system as recited in claim16 wherein said known relation is piece-wise linear between each of twoflow values among said plurality of characterizing flow rate values. 21.The system as recited in claim 17 wherein said polynomial is a quadraticpolynomial.
 22. The system as recited in claim 13 wherein said processoris further operative to determine said flow rate value at a selected oneof said selected two known speeds by: determining two flow rate valuesfrom a plurality of characterizing flow rate values at a selected aknown speed value, wherein a first one of said two flow rate values isselected corresponding to a characterizing torque value less than saidmeasured torque value and a second one said two flow rate value isselected corresponding to a characterizing torque value greater thansaid measured torque value; and determine said flow rate value byinterpolating proportionally between said first and second determinedtwo flow rate value.
 23. The system as recited in claim 1 wherein saidprocessor is operative to determine said flow rate value at a selectedone of said two known speed values by: determining a fluid flow ratevalue at said measured motor torque.
 24. The system as recited in claim13 wherein said proportional relation is a linear relation.